Method of forming prestressed tubular structures



Dec. 22, 1964 A. DAVIDSON 6 ,70

METHOD OF FORMING PRESTRESSED TUBULAR STRUCTURES Filed July 31, 1961 3Sheets-Sheet l A. DAVIDSON Dec. 22, 1964 METHOD OF FORMING PRESTRESSEDTUBULAR STRUCTURES 3 Sheets-Sheet 2 Filed July 31, 1961 A. DAVIDSON ULARSTRUCTURES Dec. 22, 1964 METHOD OF FORMING PRESTRESSED TUB Filed July31, 1961 3 Sheets-Sheet 3 United States Patent 3,162,709 METHOD 0FFORD/ENG PRESTRESSED TUBULAR STRUCTURES Alfred Davidson, American Form &Equipment C0., 30 W. Washington St., Chicago, Ill. Filed July 31, 1961,Ser. No. 127,911 Claims. (Cl. 264-228) This invention relates to moldedstructures and in particular to prestressed concrete structures and thelike. More specifically, the invention relates to prestressed concreteor plastic tubular structures such as pipe and conduit.

While concrete and similar materials have exceedingly high compressivestrength, such materials notoriously are quite low in tensile strength.Until relatively recently, the low tensile strength characteristic ofsuch materials has seriously limited the utilization thereof as it hasbeen difiicult to design structural configurations wherein tensileforces are effectively eliminated. One solution to this problem has beento prestress the concrete or similarmolded material, so that when theforces tending to place the material in tension are developed therein,they merely act against a preinduced compressive force, therebyeffectively precluding the development of tensile stresses in thematerial. The present invention is concerned with such prestressing intubular structural configurations and comprehends a new and improvedmethod of forming a prestressed tubular structure.

Thus, a principal feature of the invention is the provision of a new andimproved prestressing method.

Another feature of the invention is the provision of a new and improvedmethod of forming a prestressed tubular structure.

A further feature of the invention is the provision of such a methodincluding the steps of forming a tubular lattice-cage of high creepmaterial, helically disposing elongated members formed of a low creepmaterial coaxially on the lattice-cage, applying a longitudinal tensionstress to the elongated members, molding a tubular body with thelattice-cage and members disposed therein, maintaining the stress whilethe body sets, discontinuing application of the stress to cause themembers to apply a compressive stress in the set body, and maintainingthe structure to permit the compressive stress to cause the lattice-cageto creep and thereby distribute the compressive stress substantiallyuniformly throughout the tubular structure.

Still another feature of the invention is the provision of such a methodwherein the support is formed of a material having a high creep, or lowunit stress retaining, characteristic such as mild steel, and theelongated member is formed of a material having low creep, or higherunit stress retaining, characteristics such as high tensile steel.

Yet another feature of the invention is the provision of such a methodwherein the step of forming the latticecage comprises, disposing aplurality of rings formed of a high creep material in coaxially spacedrelationship.

Still another feature of the invention is the provision of such a methodwherein the step of forming the latticecage comprises, axially movablydisposing a plurality of rings formed of a high creep material incoaxial spaced relationship concentrically about a plurality ofcircumferentially space longitudinal members extending parallel to theaxis of said rings.

Another feature of the invention is the provision of such a methodwherein the step of forming the latticecage comprises, axially movablydisposing a plurality of rings formed of a high creep material incoaxial spaced relationship.

Yet another feature of the invention is the provision 3,162,709 PatentedDec. 22, 1964 of such a method wherein the step of forming thelatticecage comprises, axially movably disposing a plurality of ringsformed of a high creep material in coaxial spaced relationship, each ofthe rings having a small hole therethrough extending parallel to theaxis of the ring, and the step of disposing the elongated membercomprises, extending the member seriatim through the holes of the rings.

A further feature of the invention is the provision of such a methodwherein the step of forming the latticecage comprises, fixedly securinga plurality of rings formed of a high creep material in coaxial spacedrelationship concentrically about a plurality of circumferentiallyspaced longitudinal members extending parallel to the axis of the rings.

A still further feature of the invention is the provision of such amethod wherein the tension stress is applied concurrently with thedisposing of the members on the lattice-cage.

Still another feature of the invention is the provision of such a methodwherein the members are firstly disposed in parallel rectilinearrelationship concentrically about the lattice-cage, a first selectednumber of the members are wrapped about the lattice-cage by moving oneportion of each member of the number at one end of the lattice-cage inone circumferential direction annularly about the axis relative to anopposed portion of the respective members of the number at the other endof the lattice-cage, and a second selected number of the members arewrapped about the lattice cage by moving one portion of each member ofthe second number at one end of the lattice-cage in an oppositecircumferential direction annularly about the axis relative to anopposed portion of the respective members of the second number at theother end of the lattice-cage.

Another feature of the invention is the provision of such a methodfurther including a step of providing antifriction means at the pointsof contact of the members and the lattice-cage.

Yet another feature of the invention is the provision of such a methodfurther including a step of providing lubricating material at the pointsof contact of the members and the lattice-cage.

Other features and advantages of the invention will be apparent from thefollowing description taken in connection with the accompanying drawingwherein:

' 1G. 1 is an isometric view illustrating one method of wrapping anelongated tension member helically on a lattice-cage;

FIG. 2 is an isometric view illustrating another method of wrapping theelongated member helically on the latticecage;

FIG. 3 is an isometric view illustrating still another method ofwrapping a plurality of elongated members helically in circumferentiallyspaced relationship on the lattice-cage;

FIG. 4 is a perspective view illustrating another method of arrangingthe elongated members in a helical configuration in association with aplurality of supporting rings;

FIG. 5 is a perspective view illustrating another method of arrangingthe elongated members in helical configurationsin association with alattice-cage support;

FIG. 6 is a diametric section illustrating the prestressed tubularstructure with the elongated members being retained by externalrestraint means, and illustrating the arrangement of the forms as usedin molding the concrete or plastic material about the support andtension members;

FIG. 7 is a diametric section of the resultant tubular structure;

FIG. 8 is an enlarged transverse section taken substantially along-theline 88 of FIG. 7;

FIG. 9 is a transverse section similar to that of FIG. 8 butillustrating \a modified resultant structure having a plurality ofparallel longitudinally extending reinforcing rods concentrically withinthe prestressing members;

FIG. 10 is a perspective view illustrating still another method offorming the prestressed tubular structure including a pair oflattice-cages and two sets of helical prestressing members; and

FIG. 11 is a perspective view illustrating yet another method ofarranging the tension members in association with the lattice-cage.

The present invention comprehends a method of forming a prestressedtubular structure wherein the prestressing members are arranged in anopen helical configuration thereby providing preinduced stresses in thetubular structure which are directed both longitudinally andcircumferentially thereof. The prestressing means is effectively fullyembedded within the body of molded material such as concrete, plastic,or the like and, thus, is protected from external damaging forces,atmospheric corrosion and environmental chemical attack. The applicationof the tensile forces to the prestressing members may be effectedconcurrently with the arrangement thereof in the helical configurationor may be effected subsequent thereto; however, in each instance, theapplication of the tensile forces acting both longitudinally andcircumferentially may be effected as a single step in the process. Bymaintaining the tension force in the tensioning members during the timethe molded material sets, a strong bond between the tensioning membersand the molded material is effected, permitting a compressive force tobe transmitted to the molded material when the tension force applied tothe elongated members is released subsequent to the setting of themolded material.

More specifically, the invention comprehends forming the prestressedtubular structure by firstly forming a support, such as support 10, asillustrated in FIG. 1. The support includes a plurality of rings 11coaxially spaced to define a generally tubular lattice-cage which isrelatively light and open. As shown in FIG. 1, the latticecage 10 may beprovided with a plurality of longitudinally extending elements 12 towhich the rings 11 may be fixedly secured. The lattice-cage servesprimarily as a support for the prestressing members 13 comprisingelongated rods or wires wnapped helically about the lattice-cage 10.Preferably, the prestressing members 13 are wrapped in a large pitchhelix, herein having a pitch greater than the radius of the helix. InFIG. 1, one such prestressing member 13 is shown as being wrappedhelically about the lattice-cage 10 by rotating a disc 14 about the axisof the helix and lattice-cage, one end 15 of the prestressing member 13being secured to the disc 14, and the other end 16 of the prestressingmember 13 being secured to a fixed support such as a disc 17. To guidethe prestressing member 13 as it is wrapped about the lattice-cage 10, aguide arm 18 may be associated with the disc 14. At the same time thatthe disc 14 is being rotated, it is urged axially to the left as seen inFIG. 1 to provide a high tension force in the prestressing member 13.The rotating and force applying means being diagrammatically illustuatedat 19. Alternatively, as shown in FIG. 2, the force applying means maycomprise a drum 20 driven by a suitable drive 21 and the guide 18 may bedriven by a suitable separate rotating device 22.

In FIGS. 1 and 2, a single prestressing member 13 is illustnated; it isunderstood, of course, that a suitable plurality of such helicallyarranged prestressing members may be provided on the lattice-cage 10 asdesired. In FIG. 3, a method of simultaneously winding a plurality ofprestressing members 13 on the lattice-cage 10 is shown to comprise amethod generally similar to that of FIG. 1, but wherein the left-handend of each of the prestressing members 13 is secured to the disc 14 forconcurrent rotation thereof about the axis of the lattice-cage, therighthand end of the prestressing members 13 being simultaneouslyaffixed by the fixed disc 17 It should be noted that the lattice-cage Itin FIG. 3 may be provided with rings 11 which are movable on thelongitudinal members 12. Also illustrated in FIG. 3 is the arrangementof the inner core 23 and outer shell 24 defining the forms in which themolding material is poured to embed the lattice-cage 1t and prestressingmembers 13. As shown in FIG. 6, the discs 14 and 17 are arranged tomaintain the prestressing member 13 under substantial tension while thematerial, such as concrete, sets in the forms.

In applying the tensile force to the prestressing members, someconstriction of the lattic-cage 10 occurs. When the tensile force isdiscontinued, the rings will expand, thereby inducing stresses in therings 11 tending to oppose the inwardly directed prestressing forces ofthe members 13. Toward the end of effectively eliminating thisundesirable effect, it is preferred that the latticecage rings 11 beformed of a mild steel having a high creep characteristic whereby therings tend to become set in the constricted configuration after a shortperiod of time. On the other hand, it is preferred that the prestressingmembers 13 be formed of a high tensile steel having a low creepcharacteristic so that these members will continue to exert prestressforces on the concrete during the life of the structure.

As illustrated in FIG. 7, the completed structure 25 is obtained byremoving the tension applying means subsequent to a setting of theconcrete. By setting the concrete with the prestressing members 13embedded therein, a bond between the concrete and the prestressingmembers iseffected, assuring a transmission of the prestressing forcesuniformly throughout the concrete of the tubular structure 25. Means forproviding a restraint at the opposite ends of the prestressing members13 may be provided in the form of anchors 26 which may be disposedexternally of the structure 25 as shown at the upper end thereof in FIG.7 or disposed outwardly of discs 14 and 17 as shown in FIG. 6, orinternally thereof as shown at the lower end of the structure 25 in FIG.7. To provide a further improved bond between the molded material andthe prestressing members 13, suitable adhesive material as shown at 27may be provided on the prestressing members.

As seen in FIG. 8, the lattice-cage 10 and the prestressing members 13are effectively fully embedded within the molded material C in thecompleted structure 25. Thus, the prestressing members are effectivelyprotected from the atmosphere and damage from external sources.

When the tension force is released subsequent to the setting of themolded material, the prestressing members 13, by virtue of their bond tothe molded material, apply a substantial compressive force theretowhich, as a result of the helical configuration of the prestressingmembers, prestresses the molded material both longitudinally andcircumferentially in the tubular structure 25. To provide additionallongitudinal prestressing, additional prestressing members 2 8 may beprovided concentrically within the lattice-cage 10, as shown in FIG. 9.Further, by releasing the tension force on the longitudinal members 28prior to the release of the tension force on the prestressing members13, an improved bond between the prestressing members 13 and thematerial C is obtained.

Referring now to FIGS. 4, 5, 10 and 11 the invention further comprehendsa number of modified methods of forming the helical prestressing memberarrangements. As shown in FIG. 4, the support 10 may comprise aplurality of rings 11 retained in coaxial spaced relationship by asuitable means such as a split mandrel 29. The prestressing members maybe arranged in a helical configuration about the rings 11 whicheffectively define a tubular lattice-cage and may be slidingly connectedto the rings 11 by suitable ties 30. Thus, the prestressing members 13may be arranged in the helical configuration prior to the application oflongitudinal tensile stresses in the members 13, permitting the splitmandrel 29 to be removed subsequent to the installation of the members13 thereon. The longitudinal tensile forces may then be applied to theopposite ends of the prestressing members 13, movement between theprestressing members 13 and the rings 11 being permitted by the ties 30to accommodate the resultant displacement of the prestressing members.

In FIG. 5, a modified lattice-cage 31 is shown to comprise a pluralityof rings 32 retained in fixed coaxial spaced relationship bylongitudinal members 12. The rings 32 are provided with a plurality ofcircumferentially spaced holes 33 extending parallel to the axis of therings, through which the prestressing members 13 may be threaded in ahelical configuration. The holes 33 are preferably slightly larger thanthe diameter of the prestressing members 13 so that free movement of theprestressing members through the rings may be effected to permit therepositioning thereof upon the application of the longitudinal tensionforces subsequent to the threading of the members through the rings.

In FIG. 11, the prestressing members 13 are wrapped helically about alattice-cage 34 which is generally similar to lattice-cage 10. The rings11 may be axially movable on the longitudinal members 12 and the endrings 35 fixedly secured to the ends of the longitudinal members 12.Thus, the rings 11 may move during the tensioning of the prestressingmembers 13, permitting the tension ing to be effected subsequent to thehelical wrapping. Alternatively, the rings 11 may be fixedly secured tothe longitudinal members 12. throughout.

In the arrangements of each of FIGS. 4, 5 and 11, the helical wrappingis preferably effected in complementary pairs of members 13 wherein therespective members are wrapped in opposite directions, i.e., one of themembers may be wrapped in a right-hand helix and the other may bewrapped in a left-hand helix. Thus, by applying the tension force to thepaired members 13, when the rings 11 are loose, the rings will beretained by the respective members against rotation notwithstandingtheir free retention.

To provide improved free movement of the members over the rings, duringthe application of the longitudinal tensile force, anti-friction meansmay be provided; herein suitable lubricant 36 is provided at the pointof contact between the prestressing members and the rings of thesupport.

Referring now to FIG. 10, still another arrangement of the prestressmeans is shown to comprise a pair of concentrically relatedsupport-prestressing member assemblies 3'7 and 33 each of which isgenerally similar to the arrangement illustrated in FIG. 3. Assembly 38is slightly smaller than assembly 37 and is coaxially disposed withinassembly 37. As shown in FIG. 10, the assemblies may be formed byconcurrent wrapping of the prestressing members 13 of each assembly asdisclosed relative to FIG. 3.

In illustrating the invention, the molded material C has been describedas formed in place about the assembled support and helically arrangedprestressing members as by pouring the plastic molding material intosuitable forms placed therearound subsequent to the assembly thereof. Itis to be understood that other methods of embedding the prestressingassemblies in the molding material may be utilized in conjunction withthe invention. Illustratively, the molding material may be extruded soas to embed the assembly therein, or it may be centrifugally spun toembed the assembly therein. Where the material is extruded, both theinner form 40 and outer form 41 are completely omitted and where thematerial is spun, the inner form 40 may be omitted.

Each of the prestressing members defines an open relatively large pitchhelix. Longitudinal stability of the helix is provided by the lightstructural support which permits relatively free longitudinal movementof the prestressing members and limited reduction in the helix diameter.The individual prestressing members may be selectively wrapped in thesame direction on the support as where the support is rigidly assembled,or may be wrapped in opposing pairs of opposite-hand helices as wherethe support rings are loosely retained. Further, all of the members maybe concurrently twisted into a helical configuration where arangedinitially in axial parallel, circumferentially spaced relationship. Thesupport and prestressing members are effectively bonded within themolding material for improved transfer of the prestressing forces to thematerial and for improved protection of the prestressing members in use.The tensioning forces may be applied while wrapping the helices or maybe applied subsequent thereto as discussed above relative to thedifferent embodiments of the invention. Desirable control of thepretensioning forces may be effected by suitably selecting the number ofprestressing members to be utilized in the structure, the open pitcharrangement of the prestressing member helices permitting a widevariation in this selection.

As will be obvious to one skilled in the art, the invention comprehendsan improved method of forming prestressed structures which may beutilized with both solid and hollow structures. Thus, the term tubularas used herein is to be taken to include both such solid as well ashollow structures. Further, it is to be understood that the lattice-cagediscussed above may have other configurations as well as the spaced ringand ring and longitudinal member structural combinations; -i.e., one ormore spiral elements may be substituted for the ring or ring andlongitudinal member structures to provide both the circumferential andlongitudinal support means.

While I have shown and described certain embodiments of my invention, itis to be understood that it is capable of many modifications. Changes,therefore, in the construction and arrangement may be made withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

I claim:

1. The method of forming a prestressed tubular structure comprising thesteps of: forming a tubular latticecage of high creep material;helically disposing elongated members formed of a low creep materialcoaxially on the lattice-cage; applying a longitudinal tension stress tothe elongated members; molding a tubular body with the lattice-cage andmembers disposed therein; maintaining said stress while the body sets;discontinuing application of said stress to cause said members to applya compressive stress in the set body; and maintaining the structure topermit said compressive stress to cause the latticecage to creep andthereby distribute the compressive stress substantially uniformlythroughout the tubular structure.

2. The method of forming a prestressed tubular structure of claim 1wherein the step of forming the latticecage comprises, disposing aplurality of rings formed of a high creep material in coaxially spacedrelationship.

3. The method of forming a prestressed tubular structure of claim 1wherein the step of forming the latticecage comprises, axially movablydisposing a plurality of rings formed of a high creep material incoaxial spaced relationship concentrically about a plurality ofcircumferentially spaced longitudinal members extending parallel to theaxis of said rings.

4. The method of forming a prestressed tubular structure of claim 1wherein the step of forming the latticecage comprises, axially movablydisposing a plurality of rings formed of a high creep material incoaxial spaced relationship.

5. The method of forming a prestressed tubular structure of claim 1wherein the step of forming the latticecage comprises, axially movablydisposing a plurality of rings formed of a high creep material incoaxial spaced relationship, each of said rings having a small holetherethrough extending parallel to the axis of the ring, and the step ofdisposing the elongated member comprises, ex-

tending said member seriatim through the holes of said rings.

6. The method of forming a prestressed tubular structure of claim 1wherein the step of forming the latticecage comprises, fixedly securinga plurality of rings formed of a high creep material in coaxial spacedrelationship concentrically about a plurality of circumferentiallyspaced longitudinal members extending parallel to the axis of saidrings.

7. The method of claim 1 including the further step of providinganti-friction means at the points of contact of said members and saidlattice-cage.

8. The method of claim 1 including the further step of providinglubricating material at the points of contact of said members and saidlattice-cage.

9. The method of forming a prestressed tubular struc ture of claim 1wherein the tension stress is applied concurrently with the disposing ofthe members on the latticecage.

10. The method of claim 1 wherein said members are firstly disposed inparallel rectilinear reiationship concentrically about saidlattice-cage, a first selected number of said members are wrapped aboutthe lattice-cage by moving one portion of each member of said number atone end of the lattice-cage in one circumferential direction annularlyabout the axis relative to an opposed portion of the respective membersof said number at the other end of the lattice-cage; and a secondselected number of said members are Wrapped about the lattice-cage bymoving one portion of each member of said second number at one end ofthe latticecage in an opposite circumferential direction annularly aboutthe axis relative to an opposed portion of the respective members ofsaid second number at the other end of the lattice-cage.

References Cited in the file of this patent UNITED STATES PATENTS1,267,835 Zwicker May 28, 1918 2,001,237 Bille May 14, 1935 2,191,025Mitchell Feb. 20, 1940 2,303,394 Schorer Dec. 1, 1942 2,378,584 SchorerJune 19, 1945 FOREIGN PATENTS 1,107,465 Germany May 25, 1961 208,958Great Britain Jan. 3, 1924

1. THE METHOD OF FORMING A PRESTRESSED TUBULAR STRUCTURE COMPRISING THESTEPS OF: FORMING A TUBULAR LATTICECAGE OF HIGH CREEP MATERIAL;HELICALLY DISPOSING ELONGATED MEMBERS FORMED OF A LOW CREEP MATERIALCOAXIALLY ON THE LATTICE-CAGE; APPLYING A LONGITUDINAL TENSION STRESS TOTHE ELONGATED MEMBERS; MOLDING A TUBULAR BODY WITH THE LATTICE-CAGE ANDMEMBERS DISPOSED THEREIN; MAINTAINING SAID STRESS WHILE THE BODY SETS;DISCONTINUING APPLICATION OF SAID STRESS TO CAUSE SAID MEMBERS TO APPLYA COMPRESSIVE STRESS IN THE SET BODY; AND MAINTAINING THE STRUCTURE TOPERMIT SAID COMPRESSIVE STRESS TO C AUSE THE LATTICECAGE TO CREEP ANDTHEREBY DISTRIBUTE THE COMPRESSIVE STRESS SUBSTANTIALLY UNIFORMLYTHROUGHOUT THE TUBULAR STRUCTURE.